Silver Mog Pell

Are none of the redeemable items sendable in delivery box? O.o

none

About the only possible good thing you could do with a LV1 mule holding some Silver Mog Pells is turn them into Seals and use them to get others into the Battlefields during a Addition Battlefield Spoils campaign. https://www.bg-wiki.com/bg/Category:Adventurer_Campaigns#Additional_Seal_Battlefield_Spoils_Campaign

Even then, it's a lot of extra effort getting a LV1 mule to Jeuno for an orb (Use Unity warp to save time), getting them out to a battlefield, and then you will also need someone capable of killing stuff in said battlefield for a shot at Phalanx, Utsusemi Ni, or other valuable scrolls.

The only other possible use is getting an ambuscade voucher, taking the mule to Mhaura, getting an armor piece, and then taking it back to the NPC that reclaims LV119 armor for points towards copper vouchers (and a single piece won't even get you an entire copper voucher) so yeah. Not exactly worth the effort.

Since they won't retroactively add possible things to the Silver Voucher list, if neither of the above sound feasible to you, then you may as well drop the Silver Vouchers... or hold on to them in case, for whatever reason, you get a wild urge to level your mules someday... which is actually a viable time-waster if you're into doing the story over for nostalgia or want to be able to sell Mars/Venus orbs later.

Edit: At this time, it takes so little effort to get a LV1 mule to 99, it may actually be worth the half-a-day or few days (depending on your play style) it takes. https://www.reddit.com/r/ffxi/comments/57u6u6/planning_speedrun_to_level_99/

Get 2 Ambuscade seals and leech 12m
Easily do vd w/ 5

I didn't even notice the valuable seal in there. Sneaky sneaky.